I'm sure your referring to last season when Oklahoma lost to Kansas State in the Big XII championship game and then was still ranked no. 1 in the BCS and played for a piece of the National Championship. No. 2 LSU beat the Sooners 21-14. No. 3 USC (no. 1 in AP and UE) won the Rose Bowl over Michigan, 28-14.
In 2001, Nebraska lost their regular season final to Colorado. Colorado won the divisional tie-breaker and went to the Big XII championship game and upset Texas. Nebraska was still ranked No. 2 in the final BCS and played for the National Championship. No. 1 Miami beat the Cornhuskers 37-14.
Years where National Championship Game participants were both undefeated:
1992 (1) Miami 11-0 (2) Alabama 12-0
1995 (1) Nebraska 11-0, (2) Florida 12-0
1999 (1) Fla St 11-0, (2) Va Tech 11-0
2001 (1) Miami 12-0, (2) Ohio State 13-0
Note: The Bowl Coalition was formed in 1992, and lasted until 1994. In 1995, the Bowl Alliance replaced the coalition. It was replaced in 1998 by the Bowl Championship Series.
Note2: Both Nebraska and Penn State were undefeated in 1994, but because the Big11 and Pac10 were not part of the Bowl Coalition, Penn State played in the Rose Bowl. Both won bowl games, however, Nebraska finished 1st in both polls.